1. Technical Field
The present disclosure is directed to advantageous machining techniques and associated machining apparatus. The present disclosure is also directed to advantageous screw thread designs that may be efficiently and reliably fabricated using the disclosed machining techniques and/or machining apparatus. More particularly, the present disclosure is directed to machining techniques and associated machining apparatus that include multiple thread whirling heads, e.g., two thread whirling heads, associated with a single machining assembly, such that each of the thread whirling heads is able to act on a workpiece during a single, uninterrupted machining process.
2. Background Art
Screws and bolts find wide-ranging applications for attaching and/or securing elements, e.g., securing a first member with respect to a second member. Based on the diverse applications of such products, many designs have been developed. With particular focus on screw products, attention has been focused on screw thread designs and methods for fabrication thereof.
One particular area of interest for screw design and fabrication technologies is the orthopedic field. Given the clinical requirements of orthopedic screws, product performance and reliability are of tantamount importance. Titanium bone screws are used for spinal corrective surgery, trauma, and other types of bone repair and correction. Other bone screws are made of 316 stainless steel. Manufacture of bone screws generally involves strict requirements as to tolerances, surface properties, cleanliness and packaging. Titanium screws may generally range in length from 6 mm to 80 mm and have outer diameters (ODs) from 2 mm to 8.5 mm, although screws of greater length, e.g., 150 mm, shorter length, e.g., 2 mm, greater diameter, e.g., 16 mm, and lesser diameter, e.g., 1 mm, are also known. Typical bone screws require a 0.4-micron to 0.8-micron surface finish and dimensional tolerance of +/−0.025 mm. In certain applications, anodized coatings may be applied for color coding different sizes and types of screws.
In manufacture of a typical bone screw, the manufacturing operations include thread whirling, broaching, gundrilling and turning/milling. To achieve these manufacturing steps, it may be necessary to employ three or four separate machines. Two or more of the manufacturing operations may be combined using Computer Numerical Control (CNC) technology, i.e., a programmable machine tool that uses computer control technologies. CNC tools permit machine tool movements to be controlled so as to efficiently and reproducibly produce manufactured parts/products. Indeed, the manufacture of orthopedic screws generally involves a series of repetitive motions and CNC technology reduces those specific motions to computer code that guides/controls tool operations. Exemplary CNC machining tools are available under the DECO tradename from Tornos SA (Moutier, Switzerland). For purposes of Tornos machines, CNC control technology is referred to as “parallel numerical control” (PNC).
The ten-axis DECO machines can use two turning tools at the same time, completing rough and finish cuts in the same operation. One of the machine's cross slides accepts up to four live tools for operations, such as cross milling and off-center drilling. A gundrilling and high-pressure coolant attachment can be mounted on the end-working unit. Polygon milling of flats or contours can be accomplished using the machine's optional C axis on the main spindle. While the bar in the main spindle is machined, operations may be performed on the previously parted piece mounted on the counter spindle. A workpiece mounted in the pickoff spindle of a DECO machine may be typically worked by three (3) cross drilling or turning tools and by as many as six (6) end working tools.
Despite efforts to date, a need remains for improved orthopedic screw designs, manufacturing equipment and methods for fabrication of orthopedic screws. In particular, a need remains for improved bone screw designs and fabrication methods therefor. These and other needs are satisfied by the present disclosure.